Polarizing converters are well known. Commonly available film polarizers are generally effective for polarizing light but suffer from several disadvantages. The disadvantages include low efficiency (i.e., approximately 43% transmission of unpolarized incident light) and poor resistance to high temperatures (caused in part by the absorption of one-half of the unpolarized incident light). Moreover, high temperatures frequently arise in some display applications such as liquid crystal display (LCD) image projectors in which high intensity light sources are used to project images onto display screens. As a design dilemma, the low light transmission of common film polarizers require ever brighter, and hence hotter, light sources that increase the likelihood of causing heat damage to the polarizers.
Various polarizing converters of higher efficiency are also known, including those described in U.S. Pat. Nos. 5,493,624, 5,276,410, 5,539,845, 5,299,036, 5,272,496, 5,200,843, 5,164,854, 5,566,367, and 4,437,099. Many of these polarizing converters (commonly called polarization beam splitters) utilize combinations of discrete prisms and reflective surfaces arranged along relatively complex optical paths. While providing improved efficiency over common film polarizers, such polarizing converters suffer from the disadvantages of being structurally complex, bulky, and impractical to manufacture. These disadvantages are particularly troublesome in LCD projectors and flat panel display applications where structural complexity and bulk are antithetical to the design objectives of the display systems.
In accordance with the present invention, a high efficiency polarizing converter receives generally collimated light of arbitrary polarization and converts it to light of a selected polarization with minimal loss or absorption of the light. In one embodiment, the polarizing converter has a generally planar configuration that is suitable for flat panel displays, LCD projectors, and other display applications.
In its generally planar embodiment, the polarizing converter includes a planar beam splitter that has an array of light splitting prismatic structures and is analogous to a linear Fresnel lens. The planar beam splitter splits the incident light and directs it in two directions at equal but opposite deflection angles. A reflective polarizer (e.g., film) receives light propagating past the beam splitting prismatic structures. The reflective polarizer receives light of arbitrary polarization, transmits light components of a selected first polarization sense, and reflects light of an orthogonal second polarization sense.
Transmitted light of the first polarization sense passes through the reflective polarizer to a planar light combiner having an array of light combining prismatic structures that collimate and direct the light in an illumination direction that is substantially the same as the direction of the incident light. The light combiner is also analogous to a linear Fresnel lens. The light of the second polarization sense reflected by the reflective polarizer is directed back toward recesses between the prismatic structures of the beam splitter. A reflector arrangement has a planar array of frusto-prismatic reflector structures that are aligned with and extend into the recesses between adjacent prismatic structures of the beam splitter. The reflector structures reflect the light of the second polarization sense and direct it back toward the reflective polarizer. The reflection by the reflector structures changes the light from the second to the first polarization sense, so the light now passes through the reflective polarizer and into the combiner to be collimated and directed in the illumination direction.
This polarizing converter polarizes light with estimated efficiencies of up to 80 percent, representing an improvement of about 85 percent over the performance of conventional film polarizers. The generally planar configuration of the polarizing converter provides a compact, rugged, manufacturable structure that is compatible with various display applications including LCD projector displays. As a result, the polarization converter of this invention overcomes the losses or inefficiencies of conventional film polarizers while also overcoming the complexity and bulk of known high efficiency polarizing converters.
The foregoing and other features and advantages of preferred embodiments of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.